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Abstract:

A method for investigating a signal in an OFDMA transmission is disclosed.
The method comprises receiving the OFDMA transmission and obtaining a
resource grid comprising resource elements of the transmission;
determining a set of pairs of the resource elements, wherein the resource
elements of the pair are disjoint; for each pair, comparing signals of
the resource elements of the pair; determining a statistical test value
based on the comparisons; and processing the statistical test value to
obtain a desired output about the signal. A computer program for
implementing the method, a receiver arrangement adapted to perform such
investigation, and a communication apparatus using such receiver
arrangement are also disclosed.

Claims:

1. A method for investigating a signal in an OFDMA transmission, the
method comprisingreceiving the OFDMA transmission and obtaining a
resource grid comprising resource elements of the
transmission;determining a set of pairs of the resource elements, wherein
the resource elements of the pair are disjoint;for each pair, comparing
signals of the resource elements of the pair;determining a statistical
test value based on the comparisons; andprocessing the statistical test
value to obtain a desired output about the signal.

2. The method according to claim 1, wherein the processing of the
statistical test value comprisescomparing the statistical test value with
a threshold associated with a quantile for a statistical distribution of
the statistical test value; andif the statistical test value is below the
threshold, consider the signals of the resource elements of the pairs as
present, orif the statistical test value is above the threshold, consider
the signals of at least some of the resource elements of the pairs as
missing.

3. The method according to claim 2, wherein the statistical test value is
a = 1 n z 1 a - z 2 a 2 ##EQU00008## where
z1a is a product of the received signal and the complex conjugate of
an expected reference signal for the signal of one of the resource
elements of pair a, a is 1 . . . n, z2a is a product of the received
signal and the complex conjugate of an expected reference signal for the
signal of the other resource element of the pair a, and n is the number
of pairs used for comparisons.

4. The method according to claim 3, wherein the statistical distribution
is σ.sup.2.chi.2(2n).

5. The method according to claim 2, wherein the statistical test value is
F = a = 1 n z 1 a - z 2 a 2 2 n
b = 1 m z 1 b ' - z 2 b ' 2 2 m
##EQU00009## where z1a is a product of the received signal and the
complex conjugate of an expected reference signal for the signal of one
of the resource elements of pair a, a is 1 . . . n, z2a is a product
of the received signal and the complex conjugate of an expected reference
signal for the signal of the other resource element of pair a, and n is
the number of pairs used for comparisons, z'1b is a product of the
received signal and the complex conjugate of an expected reference signal
for the signal of one of the resource elements of pair b, b is 1 . . . m,
z'2b is a product of the received signal and the complex conjugate
of an expected reference signal for the signal of the other resource
element of pair b, and m is the number of pairs in a second set of pairs
of resource elements selected from the resource grid such that their
signals are known to be present.

6. The method according to claim 5, wherein the statistical distribution
is an F-distribution with 2n,2m degrees of freedom.

7. The method according to claim 2, wherein the statistical test value is
|Z1|2+|Z2|2-2|Z.sub.1.sup.HZ2|, where Z1
and Z2 are vectors of length n with elements z1a and z2a,
respectively, where n is the number of pairs, z1a is a product of
the received signal and the complex conjugate of an expected reference
signal for the signal on one of the resource elements of pair a, z2a
is a product of the received signal and the complex conjugate of an
expected reference signal for the signal of the other resource element of
pair a, a is 1 . . . n, and superscript H denotes Hermitian transpose.

8. The method according to claim 7, wherein the statistical distribution
is approximately (σ.sup.2.chi.2(2.sub.n-2).

9. The method according to claim 2, wherein the statistical test value is
F = Z 1 2 + Z 2 2 - 2 Z 1 H Z 2 2
n - 2 Z 1 ' 2 + Z 2 ' 2 - 2 Z 1 'H Z 2 '
2 m - 2 ##EQU00010## where Z1 and Z2 are vectors
of length n with elements z1a and z2a, respectively, where n is
the number of pairs, z1a is a product of the received signal and the
complex conjugate of an expected reference signal for the signal on one
of the resource elements of pair a, z2a is a product of the received
signal and the complex conjugate of an expected reference signal for the
signal of the other resource element of pair a, a is 1 . . . n, Z'1
and Z'2 are vectors of length m with elements z'1b and
z'2b, respectively, where m is the number of pairs, z'1b is a
product of the received signal and the complex conjugate of an expected
reference signal for the signal of one of the resource elements of pair
b, b is 1 . . . m, z'2b is a product of the received signal and the
complex conjugate of an expected reference signal for the signal of the
other resource element of pair b, and m is the number of pairs in a
second set of pairs of resource elements selected from the resource grid
such that their signals are known to be present, and superscript H
denotes Hermitian transpose.

10. The method according to claim 9, wherein the statistical distribution
is an F-distribution with 2n-2,2m-2 degrees of freedom.

11. The method according to any of claims 2 to 10, wherein determining any
of the pairs comprises determining reference signals associated to a
certain transmit antenna port such that the consideration of present and
missing signals, respectively, is an estimate of number of transmit
antennas used at a transmitting station making the transmission.

12. The method according to any of claims 1 to 11, wherein the
determination of the statistical test value is made for each possible
signal of the resource elements, and the processing comprises determining
the statistical test value for the possible signal having the smallest
value, wherein the received signal is considered that possible signal.

13. The method according to claim 12, wherein the signals of the resource
elements of the pairs comprises a secondary synchronisation signal short
code set, and the determining of the pairs comprises selecting the
signals of the resource elements of the pairs from one OFDM symbol.

14. The method according to claim 13, wherein signals of the resource
elements of each pair are selected from adjacent subcarriers.

15. The method according to claim 13, wherein signals of the resource
elements of each pair are selected from subcarriers that are two
subcarriers apart.

16. The method according to any of the preceding claims, further
comprising substituting the signal of a resource element of any of said
pairs with an average signal of at least two other signals of resource
elements before the comparison.

17. A computer program comprising computer program code comprising
instructions to cause a processor on which the computer program code is
executed to perform the method according to any of claims 1 to 16.

18. A receiver arrangement comprisinga receiver arranged to receive an
OFDMA transmission and obtain a resource grid comprising resource
elements of the transmission; and asignal processor for investigating a
signal in the OFDMA transmission, the signal processor being arranged
todetermine a set of pairs of the resource elements, wherein the resource
elements of the pair are disjoint;compare, for each pair, signals of the
resource elements of the pair;determine a statistical test value based on
the comparisons; andprocess the statistical test value to obtain a
desired output about the signal.

19. The receiver arrangement according to claim 18, further comprising a
comparator arranged tocompare the statistical test value with a threshold
associated with a quantile for a statistical distribution of the
statistical test value; andif the statistical test value is below the
threshold, consider the signals of the resource elements of the subsets
as present, orif the statistical test value is above the threshold,
consider the signals of at least some of the resource elements of the
subsets as missing.

20. The receiver arrangement according to any of claim 18 or 19, further
comprising an estimator arranged to estimate a number of transmit
antennas used at a transmitting station making the transmission by
determining reference signals associated to a certain transmit antenna
port such that the consideration of present and missing signals,
respectively, form the estimate of used transmit antennas.

21. The receiver arrangement according to any of claims 18 to 20, further
comprising a signal determinator arranged to make the statistical test
value for each possible signal of the resource elements, and determine
the statistical test value for the possible signal having the smallest
value, wherein the received signal is considered that possible signal.

22. A communication apparatus comprising a receiver arrangement according
to any of claims 18 to 21.

Description:

TECHNICAL FIELD

[0001]The present invention relates to a method for investigating a signal
in an OFDMA transmission, a computer program for implementing the method,
a receiver arrangement adapted to perform such investigation, and a
communication apparatus using such receiver arrangement.

BACKGROUND

[0002]In the forthcoming evolution of mobile cellular standards like
Global System for Mobile communications (GSM) and Wideband Code Division
Multiple Access (WCDMA), new transmission techniques like Orthogonal
Frequency Division Multiplexing (OFDM) will be used. Furthermore, in
order to have a smooth migration from existing cellular systems to a new
high capacity high data rate system in existing radio spectrum, the new
system has to be able to operate in a flexible bandwidth. A proposal for
such a new flexible cellular system is Third Generation (3G) Long Term
Evolution (3G LTE), in everyday speech LTE, which can be seen as an
evolution of the 3G WCDMA standard. This system will use OFDM as multiple
access technique, called Orthogonal Frequency Division Multiple Access
(OFDMA) in the downlink and will be able to operate on bandwidths ranging
from 1.25 MHz to 20 MHz. Furthermore, data rates up to 100 Mb/s will be
supported at the largest bandwidth. However, not only high rate services
are expected to be used in 3G LTE, but also low rate services like voice.
Since 3G LTE is designed for Transmission Control Protocol/Internet
Protocol (TCP/IP), Voice over Internet Protocol (VoIP) will be the
service carrying speech.

[0003]Another important aspect of LTE is the mobility function, hence
synchronization symbols, cell search and antenna detection procedures are
of major importance in order for the User Equipment (UE) to detect and
synchronize with other cells.

[0004]FIG. 1 schematically illustrates the frame structure of LTE. The
frame structure of LTE comprises a radio frame comprising ten sub-frames.
Each sub-frame comprises two slots. The transmission can be described
with a resource grid of sub-carriers and available OFDM symbols, as
illustrated in. FIG. 2, which illustrates the example of normal cyclic
prefix length. Each element in the resource grid is called a resource
element (RE) and each RE corresponds to one complex-valued modulation
symbol. The number of OFDM symbols per slot is seven for normal cyclic
prefix length and six for extended cyclic prefix length. A basic
scheduling unit is denoted a resource block. Thus, a resource block is
defined as seven or six consecutive OFDM symbols in the time domain and
twelve consecutive sub-carriers in the frequency domain.

[0005]To achieve high data rates, spatial division multiplexing of
multiple data streams to a single user equipment (UE) may be provided.
Two, and up to four transmit antennas can be used.

[0006]Within each downlink sub-frame, downlink control signalling is
located in the first n OFDM symbols, where n is three or less. There is
no mixing of control signalling and shared data in an OFDM symbol.
Downlink control signalling can comprise a format indicator to indicate
the number of OFDM symbols used for control in this sub-frame, scheduling
control information, and acknowledgement indicator associated with uplink
data transmission. The sub-frames also comprise reference symbols at
specific locations in time and frequency of the grid for the respective
transmit antennas.

[0007]A fundamental problem in LTE operation is to find out if a given
known signal is sent in a given set of REs, or, which particular signal
in a given set of possible signals that is sent in a given set of REs.
This problem occurs for instance when a primary synchronization signal
has been detected, and the UE needs to detect which of many possible
secondary reference signals that is transmitted next to the primary one.
Another instance of the problem occurs in so-called blind detection of a
possible second (or third and fourth) eNode B transmit antenna port. In
the presence of such an antenna port, certain known reference signals,
t1-t4 for the antenna ports, respectively, are sent as indicated above,
and may thus be used to detect the presence of a second, third or fourth
antenna port by suitable processing of the signals received in these REs.
The knowledge for the UE about the number of transmit antennas used is of
major importance for good signal power measurements, i.e. mobility
reasons, as well as for the possibility to decode the control channel,
having different coding depending on the number of transmit antennas,
once a handover to a new cell is needed.

[0008]Given is thus a set V of REs, or positions of ODFM symbols. Each
element of V is a couple (i,k), where i is the time-index and k is the
frequency-index of the RE in question. The question to answer is then
either [0009](i) was some given signal ri,k, (i,k) εV,
sent in the REs V, or [0010](ii) given a set S={r.sub.(i,k)p,
(i,k)εV}, p=1, . . . , P, of signals, which of the signals in
this set was sent in the REs V?

[0011]The standard model for signal transmission in OFDM systems is
yi,k=hi,ksi,k+ei,k, where si,k is the
transmitted signal, hi,k is the channel coefficient, yi,k is
the received signal and ei,k is noise, all in RE (i,k). All these
quantities are complex numbers; the hi,k often considered as random
variables from a time-frequency random process, i.e. random field, while
the ei,k are usually considered as independent, across time and
frequency, complex symmetric Gaussian random variables with some, in
general unknown, variance σ2.

[0012]A straightforward approach to solve either of the above questions is
to first compute an estimate hi,k of hi,k, then compute
si,k=hi,k1yi,k as an estimate of si,k and
finally compute the sum

Q=Σ.sub.(i,k)εVr*i,ksi,k

[0013]where super-index * denotes complex conjugation. If the si,k
are indeed equal to the ri,k, then the sum roughly equals
Q=Σ.sub.(i,k)εV|ri,k|2 up to some noise, and is
thus large. A large value of Q thus indicates that the signals ri,k
were in fact transmitted, i.e. answer question (i) above. If it is to be
decided on one candidate from a set of signals, i.e. question (ii) above,
the signal yielding the largest value of Q is picked. This approach is
known as coherent correlation. A problem with this approach is the
estimation of the channel coefficients hi,k. Typically it is assumed
that hi,k is constant over some span of time and frequency, i.e. a
time-frequency rectangle in the grid of REs. The estimate hi,k is
then computed for (i,k) within this rectangle as an average of ratios
yi,k/r*i,k, where the r*i,k are some known signals
transmitted within the time-frequency rectangle and the indices (i', k')
range over the REs within the rectangle where these known signals are
sent; the average forming hi,k is hence computed over the same
indices. The known signals r*i,k can be e.g. parts of the primary
synchronization signal, when it has been detected, which is placed close
to the secondary synchronization signal, or they can be reference signals
from antenna port 0, which is always present. To compute hi,k over
different indices (i, k), the rectangle moves along. In short, hi,k
is computed as a local average in the time-frequency RE domain. Sometimes
this is a weighted average, where REs close to (i,k) are given larger
weights.

[0014]Now, since the hi,k arise from a random process, they are not
constant over the time-frequency rectangle, and therefore, the local
average forming hi,k will suffer from bias as an estimator. If the
rectangle is chosen large, this bias will be large, in particular in case
of large speed and thus Doppler spread, and large delay spreads. On the
other hand, if the time-frequency rectangle is small, the variance of
hi,k will become large because the average has few terms.

[0015]Another problem with coherent correlation is that if there is a
frequency error in the local receiver's local oscillator, this will
introduce a phase shift, Δ say, per RE time unit, i.e. OFDM symbol.
This will in turn affect the value of Q above. Thus one often considers
the absolute value |Q|, rather than Q itself.

[0016]Yet another problem with coherent correlation pertains to question
(i) above; how large should Q, or |Q|, be in order to decide that the
signals si,k were sent? The appropriate threshold depends on the
transmission power and the noise level, of which at least the latter is
not known exactly. Thus it is typically problematic to determine an
appropriate threshold.

[0017]Thus, there is a need for at least one of an improved approach to
determine if a given signal is sent in a given set of OFDM symbols, i.e.
to answer question (i) above, and an improved approach to determine what
signal that was sent, i.e. to answer question (ii) above.

SUMMARY

[0018]An object of the invention is to at least alleviate the above stated
problem. The present invention is based on the understanding that

[0019]According to a first aspect, there is provided a method for
investigating a signal in an OFDMA transmission, the method comprising
receiving the OFDMA transmission and obtaining a resource grid comprising
resource elements of the transmission; determining a set of pairs of the
resource elements, wherein the resource elements of each pair are
disjoint; for each pair, comparing signals of the resource elements of
each pair; determining a statistical test value based on the comparisons;
and processing the statistical test value to obtain a desired output
about the signal.

[0020]The desired output about the signal may be an output signal
indicating if a given signal was sent in a given set of OFDM symbols,
i.e. answer to question (i) above, and/or indicating what signal that was
sent, i.e. answer to question (ii) above.

[0021]The processing of the statistical test value may comprise comparing
the statistical test value with a threshold associated with a quantile
for a statistical distribution of the statistical test value; and if the
statistical test value is below the threshold, consider the signals of
the resource elements of the pairs as present, or if the statistical test
value is above the threshold, consider the signals of at least some of
the resource elements of the pairs as missing.

[0022]The determining of any of the pairs may comprise determining
reference signals associated to a certain transmit antenna port such that
the consideration of present and missing signals, respectively, is an
estimate of the number of transmit antennas used at a transmitting
station making the transmission.

[0023]The determination of the statistical test value may be made for each
possible signal of the resource elements, and the processing comprises
determining the statistical test value for the possible signal having the
smallest value, wherein the received signal is considered that possible
signal.

[0024]The signals of the resource elements of the pairs may comprise a
secondary synchronisation signal short code set, and the determining of
the pairs may then comprise selecting signals of the resource elements of
the pairs from one OFDM symbol. The signals of the resource elements of
each pair may be selected from adjacent subcarriers, or be selected from
subcarriers that arc two subcarriers apart.

[0025]The method may further comprise substituting the signal of a
resource element of any of said pairs with an average signal of at least
two other signals of resource elements before the comparison.

[0026]According to a second aspect, there is provided a computer program
comprising computer program code comprising instructions to cause a
processor on which the computer program code is executed to perform the
method according to the first aspect.

[0027]According to a third aspect, there is provided a receiver
arrangement comprising a receiver arranged to receive an OFDMA
transmission and obtain a resource grid comprising resource elements of
the transmission; and a signal processor for investigating a signal in
the OFDMA transmission, the signal processor being arranged to determine
a set of pairs of the resource elements, wherein the resource elements of
the pair are disjoint; compare, for each pair, signals of the resource
elements of the pair; determine a statistical test value based on the
comparisons; and process the statistical test value to obtain a desired
output about the signal.

[0028]The desired output about the signal may be an output signal
indicating if a given signal is sent in a given set of OFDM symbols, i.e.
answer to question (i) above, and/or indicating what signal that was
sent, i.e. answer to question (ii) above.

[0029]The receiver arrangement may further comprise a comparator arranged
to compare the statistical test value with a threshold associated with a
quantile for a statistical distribution of the statistical test value;
and if the statistical test value is below the threshold, consider the
signals of the resource elements of the pairs as present, or if the
statistical test value is above the threshold, consider the signals of at
least some of the resource elements of the pairs as missing.

[0030]The receiver arrangement may further comprise an estimator arranged
to estimate the number of transmit antennas used at a transmitting
station making the transmission by determining reference signals
associated to a certain transmit antenna port such that the consideration
of present and missing signals, respectively, form the estimate of used
transmit antennas.

[0031]The receiver arrangement may further comprise a signal determinator
arranged to make the statistical test value for each possible signal of
the resource elements, and determine the statistical test value for the
possible signal having the smallest value, wherein the received signal is
considered that possible signal.

[0032]According to a fourth aspect, there is provided a communication
apparatus comprising a receiver arrangement according to the third
aspect.

[0033]Any of the aspects relies on forming a statistical test value which
is statistically processed. With a consideration of a proper statistical
distribution of the test value, a desired output giving information about
the signal is achievable.

[0034]The statistical test value may be

a = 1 n z 1 a - z 2 a 2 ##EQU00001##

[0035]where z1a is the product of the received signal and the complex
conjugate of an expected reference signal for the signal of one of the
resource elements of pair a, a is 1 . . . n, z2a is the product of
the received signal and the complex conjugate of an expected reference
signal for the signal of the other resource element of the pair a, and n
is the number of pairs used for comparisons. The statistical distribution
may then be σ2χ2(2n).

[0037]where z1a is the product of the received signal and the complex
conjugate of an expected reference signal for the signal of one of the
resource elements of pair a, a is 1 . . . n, z2a is the product of
the received signal and the complex conjugate of an expected reference
signal for the signal of the other resource element of pair a, and n is
the number of pairs used for comparisons, z'1b is the product of the
received signal and the complex conjugate of an expected reference signal
for the signal of one of the resource elements of pair b, b is 1 . . . m,
z'2b is the product of the received signal and the complex conjugate
of an expected reference signal for the signal of the other resource
element of pair b, and m is the number of pairs in a second set of pairs
of resource elements selected from the resource grid such that their
signals are known to be present. The statistical distribution may then be
an F-distribution with 2n,2m degrees of freedom.

[0038]The statistical test value may be
|Z1|2+|Z2|2-2|Z1HZ2|, where Z1
and Z2 are vectors of length n with elements z1a and z2a,
respectively, where n is the number of pairs, z1a is the product of
the received signal and the complex conjugate of an expected reference
signal for the signal on one of the resource elements of pair a, z2a
is the product of the received signal and the complex conjugate of an
expected reference signal for the signal of the other resource element of
pair a, a is 1 . . . n, and superscript H denotes Hermitian transpose.
The statistical distribution may then be approximately
σ2χ2(2n-2).

[0040]where Z1 and Z2 arc vectors of length n with elements
z1a and z2a, respectively, where n is the number of pairs,
z1a is a product of the received signal and the complex conjugate of
an expected reference signal for the signal on one of the resource
elements of pair a, z2a is a product of the received signal and the
complex conjugate of an expected reference signal for the signal of the
other resource element of pair a, a is 1 . . . n, Z'1 and Z'2
are vectors of length in with elements z'1b and z'2b,
respectively, where m is the number of pairs, z'1b is a product of
the received signal and the complex conjugate of an expected reference
signal for the signal of one of the resource elements of pair b, b is 1 .
. . m, z'2b is a product of the received signal and the complex
conjugate of an expected reference signal for the signal of the other
resource element of pair b, and m is the number of pairs in a second set
of pairs of resource elements selected from the resource grid such that
their signals are known to be present, and superscript H denotes
Hermitian transpose. The statistical distribution may then be an
F-distribution with 2n-2,2m-2 degrees of freedom.

BRIEF DESCRIPTION OF DRAWINGS

[0041]FIG. 1 schematically illustrates the frame structure of 3G LTE.

[0042]FIG. 2 schematically illustrates an example of a resource grid of
sub-carriers and OFDM symbols.

[0052]The idea of the invention is to partition a set V of resource
elements (REs) in which a hypothesized signal is sent as
V=V1∪V2 where V1={v11, v12, . . . ,
v1n} and V2=(v21, v22, . . . , v2n) arc disjoint
subsets of equal size n, i.e. the size of V is thus 2n, and then to make
pair-wise comparisons of the signals received in REs v1a and
v2a respectively, for a=1, 2, . . . , n. In the following
subsections, the details for questions (i) and (ii) above are given.

[0053]To answer the question `Was some given signal ri,k,
(i,k).di-elect cons.V, sent in the REs V?`, as stated in question (i)
above, an approach is to compute

z1a=r*i,kyi,k for a=1, 2, . . . , n, where (i,k)=v1a,
and

z2a=r*i,kyi,k for a=1, 2, . . . , n, where (i,k)=v2a.

[0054]That is, for each RE in V the received signal is correlated with the
possible transmitted signal, and then the results are sorted according to
the partition of V.

[0055]Under the assumed signal transmission model above, it is found that

[0056]If the transmitted signal si,k for (i, k).di-elect cons.V are
indeed equal to the hypothesized ri,k, then the above simplifies to

z1a=hi,k+r*i,kei,k for a=1, 2, . . . , n, where
(i,k)=v1a, and

z2a=hi,k+r*i,kei,k for a=1, 2, . . . , n, where
(i,k)=v2a;

[0057]here it is assumed that |ri,k|2=1 for (i,k)εV, but
the case of other moduli is easily carried back to the above by
multiplying by r*i,k/|ri,k|.

[0058]Now the partition of V is assumed to be done in such a way that for
each a=1, 2, . . . , n, the REs v1a and v2a are close in the
time-frequency domain. It then makes sense to assume, or approximate,
that the channel coefficients in these two REs agree; hi,k=hi,k
for (i,k)=v1a and (i',k')=v2a. It is then found that

z1a-z2a.di-elect cons.CN(0,2σ2),

[0059]where CN(μ, τ) denotes the complex symmetric Gaussian
distribution with mean μ and variance τ, and the fact that
r*i,kei,k.di-elect cons.CN(0,σ2) is used. This
subtraction is indeed considered as a cornerstone of the invention;
rather than estimating channel coefficients, these are cancelled out
under the assumption that the transmitted signal equals the hypothesized
one and that the channel coefficients agree within each pair being
compared.

[0060]Since the real and imaginary parts of a CN(0,2σ2) random
variable are independent real Gaussian variables with variance
σ2, it is therefore found that

|z1a-z2a|2.di-elect cons.σ2χ2(2)

[0061]where χ2 (q) is the χ2 distribution with q degrees
of freedom. Furthermore, the differences z1a-z2a are
independent across a.

[0062]If, on the other hand, si,k does not equal ri,k for some
index (i,k) involved in a pair (z1a,z2a), then there will be no
cancellation of means in the difference z1a-z2a and the square
|z1a-z2a|2 will tend to be larger than a
σ2χ2(2) random variable. Thus, using the test
statistic

C = a = 1 n z 1 a - z 2 a 2 ##EQU00004##

[0063]it can be concluded that the signal si,k was indeed sent in the
REs (i,k).di-elect cons.V if C is sufficiently small.

[0064]To decide what is "sufficiently small", it should be noticed that if
si,k=ri,k for all (i,k)εV, then
Cεσ2χ2(2n). To estimate χ2, a
statistic C' is computed in an entirely analogous fashion, but based on
signals in REs that are certain to contain a particular sequence. This
signal can be e.g. the primary synchronization signal, or reference
signal from antenna port 0, i.e. t1 as illustrated in FIG. 2, which is
always present. Assuming that this other set of signals is split into m
pairs, there is thus available a statistic
C''εσ2χ2 (2m). The ratio

F = C / 2 n C ' / 2 m ##EQU00005##

[0065]will then have an F-distribution with (2n,2m) degrees of freedom,
and the (null) hypothesis that si,k=ri,k for (i,k).di-elect
cons.V , at level α is rejected, if F exceeds the
(1-α)-quantile of the F-distribution with (2n,2m) degrees of
freedom. If F does not exceed this quantile, the hypothesis is not
rejected and it can be concluded that the signal si,k was sent.

[0066]Now consider the situation of determining which signal
ri,k.sup.(p), p=1, . . . , P, that was transmitted in
REs(i,k).di-elect cons.V, i.e. the second question stated above. This
problem is solved by computing the above statistic C for each of signal
ri,k.sup.(p), yielding numbers C.sup.(p), p=1, . . . , P. The output
of the selection procedure is then the signal whose index p has the
smallest C.sup.(P).

[0067]These findings are utilised in a method, as illustrated in FIG. 3,
for investigating a signal in an OFDMA transmission. Here, investigation
means either to find if a signal is present or missing, or to find what
signal that was transmitted in the transmission, i.e. decoding the
signal. Of course, both of these achievements can be included in the act
of investigating. The method comprises receiving the OFDMA transmission
in a reception step 300, such that the resource grid, as for example
illustrated in FIG. 2, is obtained. Pairs of disjoint REs are determined
in a signal pair determination step 302. For example, for each pair, a
first signal of one of the REs with reference signals associated with a
certain transmit antenna, and a second signal of another of the REs with
reference signals associated with the certain transmit antenna are
selected. Similarly, further pairs comprising REs with reference signals
associated with possible other transmit antennas can be formed. As a
further example, pairs can also be determined from REs with reference
signals associated with a first transmit antenna, which for some
applications are known to always be present. The determined pairs then
form basis for the investigation, which is, as elucidated above,
performed by a pair-wise comparison of resource elements, or function of
these, based on the formed pairs, e.g. each signal of the resource
elements multiplied with a complex conjugate of an expected or possible
transmitted signal and then forming a difference z1a-z2a, in an
RE comparison step 304. One or more statistical test values are
determined based on the compared REs, as also elucidated above, in a test
value determination step 306, and then processed in a test value
processing step 308. The test value C can be based on
|z1a-z2a|2 aggregated over all pairs a, where C has a
σ2χ2 distribution. A test value F can also be based
on the two disjoint subsets determined from in pairs of signals of REs in
which a specific signal is known by the receiving entity to be present,
e.g. with reference signals associated with only the first transmit
antenna of the transmitting entity, as well as the aggregated
|z1a-z2a|2 over n pairs, forming the test value

[0068]which has an F-distribution with 2n, 2m degrees of freedom. An
advantage of this is that variance do not have to be determined
separately.

[0069]During initial cell search, the local oscillator frequency may not
be fully aligned with the actual system carrier frequency. The result of
such a frequency error is a phase shift in the received signal per OFDM
symbol time unit. Assuming that the two signals in each pair being
compared are separated by a common distance in time, a more suitable
model for the channel coefficients is then
hi,kejΔ=hi',k'for (i,k)=v1a and
(i',k')=v2a, where Δ represents the phase shift incurred by
the frequency error over the time between the two REs of each pair. In
fact, the same model can be suitable even without a frequency error, then
to some extent capturing Doppler shifts in the time direction or channel
delay in the frequency direction. Thus, according to a further
embodiment, vectors Z1 and Z2 of length n are formed, where n
is the number of pairs, and the elements of the vectors are z1a and
z2a, respectively, a=1 . . . n, and the signals of the REs are
multiplied with complex conjugates of expected or possible signals as
demonstrated above. A statistic corresponding to C above is obtained by
minimizing |Z1ejΔ-Z2|2 over
Δε[0,2π). By expanding the square
|Z1ejΔ-Z2|2 we find that it equals
|Z1|2+|Z2|2-2Re(ejΔZ2HZ1),
where superscript H denotes Hermitian transpose. Clearly, this expression
is minimized when, by choosing Δ suitably, the real part of
ejΔZ2HZ1 equals its modulus
|Z2HZ1|=|Z1HZ2|. The statistical test value
C is then |Z1|2+|Z2|2-2|Z1HZ2|. In
this case, the statistical distribution is approximated to
σ2χ2(2n-2). This approximation is found to work well
in practical applications, where it is found that the distribution of C
is reasonably well approximated by σ2χ2(2n-2),
provided the transmitted signal si,k for (i,k)εV is indeed
equal to the hypothesized ri,k. The embodiment may work satisfying
also with the distribution of C approximated by distributions similar to
σ2χ2(2n-2).

[0070]For the case of both handling a frequency error and avoiding
determination of the variance separately, the statistical test value can
be formed as

[0071]where Z1 and Z2 are vectors of length n with elements
z1a and z2a, respectively, where n is the number of pairs,
z1a is a product of the received signal and the complex conjugate of
an expected reference signal for the signal on one of the resource
elements of pair a, z2a is a product of the received signal and the
complex conjugate of an expected reference signal for the signal of the
other resource element of pair a, a is 1 . . . n, Z'1 and Z'2
are vectors of length m with elements z'1b and z'2b,
respectively, where m is the number of pairs, z'1b is a product of
the received signal and the complex conjugate of an expected reference
signal for the signal of one of the resource elements of pair b, b is 1 .
. . m, z'2b, is a product of the received signal and the complex
conjugate of an expected reference signal for the signal of the other
resource element of pair b, and m is the number of pairs in a second set
of pairs of resource elements selected from the resource grid such that
their signals are known to be present, and superscript H denotes
Hermitian transpose. Here, the threshold is based on an F-distribution
with 2n-2,2m-2 degrees of freedom.

[0072]In the test value processing step 308, the obtained one or more test
values are processed to obtain the desired information about the signal,
i.e. if the signal is present or missing, and/or what the signal is. In
the first alternative, information about the number of transmit antennas
used at transmitting end can be gained. In the latter, the signal can be
decoded. The signal can be decoded by forming test values for each
possible signal, and determining the signal to be the one of them that is
associated to the smallest test value. The test values are thus formed by
multiplying the received signal with complex conjugates r*i,k of all
possible or expected signals ri,k, following the notation above.

[0073]FIG. 4 is a flow chart illustrating an embodiment of processing of
the test value. The statistical test value is compared with a threshold,
wherein the threshold is associated with a quantile for the appropriate
statistical distribution of the statistical test value. The appropriate
statistical distribution for different statistical test values is
discussed above. Based on the comparison with the threshold, the signal
is considered present or missing, i.e. the signal is considered present
when the statistical test value is below the threshold, and missing when
above the threshold.

[0074]FIG. 5 is a flow chart illustrating an embodiment of processing of
the test values. The test values for each possible signal that may be
present arc compared, wherein the signal associated with the smallest
test value obtained is considered to be the received signal. One test
value for each possible signal is thus formed for determining the
received signal, as discussed above.

[0075]It has been demonstrated that a signal of an RE, or its function
when considering an expected or possible signal, is compared with
another. As an alternative, a virtual signal can be formed as an average
of signals of two or more REs, and then be used as the signal of the RE
in the way described above. This can be advantageous when high Doppler
effects are present. For example, instead of comparing signal of t2 of
symbol 1 of the first slot in FIG. 2 with signal t2 of symbol 1 of the
second slot, a shorter distance in time is achieved by averaging the two
signals of the t2s of symbol 5 of the first slot as illustrated in FIG. 2
and compare the formed virtual signal, i.e. the average, with signal t2
of symbol 1 of the second slot. Here, the assumption of unchanged channel
is better in a fast changing signal environment, i.e. separation of 3
OFDM symbols instead of 7. Similar ways of action are possible for the
other signals too.

[0076]In LTE, the secondary synchronization signal (SSS) is sent on 62
consecutive, in the frequency domain, REs, except for a gap in the middle
at the DC subcarrier, as illustrated in FIG. 6. In frequency domain
duplex (FDD) systems, SSS is sent every 5th subframe, and immediately
preceding the primary synchronization signal (PSS). In time domain duplex
(TDD) systems, SSS is also sent every 5th subframe but 3 OFDM symbols
after the PSS.

[0077]When the UE searches for new cells, either at start up or when
already connected but searching for neighbouring cells, it first detects
the cell's PSS to determine the basic timing of the cell, and then
detects the SSS to determine the full timing of the cell as well as the
cell identity. The SSS is a combination of two sequences, so-called short
codes, of length 31. There are 31 variants of each of them, but only 168
different combinations are actually allowed. The two short codes are
interleaved in the frequency domain, as illustrated by the differently
marked REs in FIG. 6, forming the full SSS. The SSS is sent in subframes
0 and 5 for FDD, or in the downlink pilot timeslot (DwPTS) subframes
following these slots for TDD. The interleaving of short codes alternates
between these two subframes, i.e., which short code that is sent on the
lowest-numbered subcarrier, so that detecting the SSS also enables the UE
to determine which subframe that is number 0, and hence the full timing
of the cell. In addition, the actual identity of the SSS, one of 168,
together with the three possible variants of PSS give the complete
identity of the cell.

[0078]Detection of SSS is often done by coherent correlation, which is
particularly useful when the PSS is sent immediately preceding the SSS,
as in FDD LTE. In TDD LTE the distance in time between PSS and SSS is, as
noted above, 3 OFDM symbols, which may degrade performance at high
Doppler speeds.

[0079]By making pair-wise comparisons across frequency, a procedure for
SSS detection can be constructed that is less sensitive to Doppler, as it
only uses REs during a single OFDM symbol in time, i.e. when SSS is,
potentially, sent. The pairs can be formed either by pairing adjacent, in
the frequency domain, REs, as illustrated by arcs in FIG. 7, or by
pairing REs two subcarriers apart, as illustrated by arcs in FIG. 8. In
the former case, the set of possible signals consist of the 168 possible
combinations of short codes, whereas in the latter case we detect each
short code separately from a set of 31 possible ones. The former
procedure thus involves more computations, but since the REs in each pair
are closer in frequency, it is less sensitive to channel delay spread. In
all other senses, the principles demonstrated above apply.

[0080]The methods according to the present invention are suitable for
implementation with aid of processing means, such as computers and/or
processors. Therefore, there is provided computer programs, comprising
instructions arranged to cause the processing means, processor, or
computer to perform the steps of any of the methods according to any of
the embodiments described with reference to any of FIGS. 3 to 8. The
computer programs preferably comprises program code which is stored on a
computer readable medium 900, as illustrated in FIG. 9, which can be
loaded and executed by a processing means, processor, or computer 902 to
cause it to perform the methods, respectively, according to embodiments
of the present invention, preferably as any of the embodiments described
with reference to any of FIGS. 3 to 8. The computer 902 and computer
program product 900 can be arranged to execute the program code
sequentially where actions of the any of the methods are performed
stepwise, but can also be arranged to perform the actions on a real-time
basis, i.e. actions are performed upon request and/or available input
data. The processing means, processor, or computer 902 is preferably what
normally is referred to as an embedded system. Thus, the depicted
computer readable medium 900 and computer 902 in FIG. 9 should be
construed to be for illustrative purposes only to provide understanding
of the principle, and not to be construed as any direct illustration of
the elements.

[0081]The approach can be used in a receiver arrangement 1000, as
schematically illustrated in FIG. 10. The receiver arrangement 1000
comprises receiving circuitry 1002, which down-converts and demodulates
an OFDMA transmission received via one or more antennas 1004, and outputs
resource blocks to a signal investigator 1006 of the receiver arrangement
1000. The signal investigator 1006, which preferably is a signal
processor, is arranged to perform the actions according to any of the
embodiments described with reference to any of FIGS. 3 to 8, which can be
implemented as described with reference to FIG. 9. The receiver
arrangement 1000 can also comprise a comparator arranged to compare the
statistical test value, as elucidated above, with a threshold associated
with a quantile for a statistical distribution of the test value, and
thus determine the investigated signal present if the test value is below
the threshold, and missing if above the threshold. The receiver
arrangement 1000 can also comprise an estimator arranged to estimate the
number of transmit antennas used at a transmitting station making the
OFDMA transmission by determining presence of signals associated with
antenna ports as described above. The receiver arrangement 1000 can
further comprise a signal determinator arranged to investigate
statistical test values for each possible signal, and thereby determining
the received signal as the one having smallest associated test value. Any
of the comparator, estimator, and signal determinator can be integrated
with or included in the signal investigator 1006.

[0082]The receiver arrangement 1000 can be used in a communication
apparatus 1100, as illustrated in FIG. 11. The communication apparatus
1100 can be a mobile telephone, a communication card in a computer, or
any other apparatus arranged to perform communications in an OFDMA
communication system. The communication apparatus 1100 comprises the
receiver arrangement 1000, which provides its output to higher layer
signal processing means 1102 of the communication apparatus 1100. The
communication apparatus 1100 preferably further comprises a processor
1104 arranged to control operations of the communication apparatus 1100.
The signal investigator 1006 of the receiver arrangement 1000 can he
integrated with the processor 1104, which then performs the actions
elucidated above. The processor 1104 preferably works with aid of a
memory 1106, which is arranged to store and provide work and/or content
information. Optionally, if the communication apparatus 1100 is an
apparatus to be operated directly by a user, such as a mobile phone, the
communication apparatus 1100 can comprise a user interface 1108, which
can comprise input and output means such as microphone, speaker, display
screen, keys, joystick, touchpad, touch sensitive screen, etc.